寄生虫 (parasite or maybe Kaio)

let's call him Kaio (I wasn't creative, sorry)

you had found a little reddish dirt on your bookshelf, as it was small and you couldn't remove it, you just left it there, after a while the stain got bigger and whitish spiral stripes started to appear, again you ignored it

the stain started to get bigger and bigger, but you didn't mind because you lived alone and no one visited you

until one day you saw a boy, or girl, you couldn't identify them, sitting cross-legged in the same place as the stain, they were wearing a long blouse and shorts, but the same color as your body, bright red with whitish spiral stripes

Elu looked like a puppy, he couldn't talk and his eyes were white without pupils with a medium hairline, he was half a meter tall and couldn't stand up, he made some strange sounds similar to a tantrum

He is a bit attached and dependent, he literally depends on you to feed him and even help him.

uses the pronouns Elu/Delu (neutral pronoun)

the genus of delu parasite is 8÷ (possible neuter)

He copies people, but you have no idea who he copied to look like that

He is practically a puppy and is developing.